The present invention relates generally to perovskite-phase thin films and more particularly to new perovskite phases of the form A.sub.x B.sub.y O.sub.3 as thin films and the preparation thereof.
Perovskite-phase materials are of great interest for a wide range of ferroelectric, dielectric, piezoelectric, and actuator applications, including their use in thin film capacitors and micro-electromechanical systems, due to the very high dielectric constant and the electromechanical properties. These materials are classically synthesized by sintering metal oxide mixtures at high temperatures. Lists of materials that form perovskite geometries upon sintering are ubiquitous in the literature. A good summary of perovskite-phase metal oxides is found in Landolt-Bornstein, (Numerical Data and Functional Relationships in Science and Technology, Volume 9, Ferro- and Antiferroelectric Substances, K. Hellwege, ed., 1975). The perovskite phase materials are prepared sometimes as powders and sometimes as thin films.
The perovskite-phase structure is generally described as a simple cubic structure with a cation on the corners, another cation in the body center and oxygen atoms in the center of the faces. Any structure consisting of the corner-linked oxygen octahedra with a cation filling the octahedral hole and another cation filling the dodecahedra hole is regarded as a perovskite structure represented by the general formula A.sub.x B.sub.y O.sub.3.
In these perovskite phase materials, because of the -6 valence charge associated with the oxygen atoms, the total valence charge of the metal cations A and B, where A and B may be multiple cations, must equal +6. Examples of perovskite materials, include BaTiO.sub.3, with barium having a +2 charge and titanium a +4 charge; NaTaO.sub.3, with sodium having a +1 charge and tantalum a +5 charge; NaNbO.sub.3, with sodium having a +1 charge and niobium a +5 charge; and Sr(Cu.sub.1/3 Nb.sub.2/3)O.sub.3 with strontium having a +2 charge, and the copper-niobium mix having an equivalent +4 charge (1/3 copper atom times a valence charge of +2 plus 2/3 niobium atom times a valence charge of +5 equals +4). New perovskite-phase compounds are sought because of the potential for enhanced ferroelectric properties in powder and thin film applications.
Solution routes are widely used for the production of thin films, such as A.sub.x B.sub.y O.sub.3 thin films, through spin-cast or dip-coating methodologies. These methods are typically used due to the flexibility in the stoichiometry of precursor solutions, the ease of altering processing variables, cost effectiveness (inexpensive), the reduction of the sintering temperatures, and the capability to integrate with existing semi-conductor processes.
Oda et al., (U.S. Pat. No. 4,874,598, issued on Oct. 17, 1989) and Watanabe et al., (U.S. Pat. No. 5,229,101, issued on Jul. 20, 1993) describe an aqueous solution method of producing a perovskite-type oxide of the ABO.sub.3.
Haertling and Land (Ferroelectrics, 1972, Vol. 3, pp. 269-280) describe a sol-gel method of making perovskite-type materials by mixing alkoxide solutions of zirconium and titanium with a lead oxide powder and lanthanum acetate solution, hydrolyzing the solution to form a white-colored solution slurry of paint-like consistency, drying the slurry, crushing the dried product, calcining the product and then crushing and calcining again to obtain a fine-grained powder product, from which films can be prepared.
Boyle et al. (Chem. Mater., 1997, 9, 3187-3198) describe a sol gel process for the preparation of thin films of lead magnesium niobium oxide ferroelectric materials.
Boyle (U.S. Pat. No. 5,683,614, issued on Nov. 4, 1997) describes a sol gel method of forming a bismuth-strontium-tantalum oxide (SBT) ferroelectric material using a solution route process.